1. Field of the Invention
The present invention relates to an electrophotographing apparatus for a copying machine, a laser printer, and the like. More particularly, the invention relates to an electrophotographing apparatus wherein a plurality of developing devices are arranged around a photosensitive member, and the image forming conditions are being controlled for each color in accordance with the dark attenuation characteristic of the photosensitive member.
2. Related Background Art
A photosensitive member which is generally used for an electrophotographing apparatus for a copying machine and others generates the so-called "dark attenuation", that is, if the photosensitive member is left intact after once charged by an electrostatic charger, its charged potential is lowered (the amount of the charged potential is reduced). This dark attenuation (potential reduction) is known to be increased as the time elapses. If, for example, a photosensitive member rotates, the dark attenuation will increase in accordance with the distance that the photosensitive member has traveled after the electrostatic charging.
Meanwhile, there is known an electrophotographing apparatus which is provided with a plurality of developing devices arranged around the photosensitive member. A full color copying machine having four developing devices for four color toners, each in cyan, magenta, yellow, and black, can be cited in this respect, for example. The above-mentioned four developing devices are arranged around the photosensitive member in different positions along its rotational direction thereby to apply the toners in each color to the corresponding latent images sequentially. Therefore, the distances to each of the developing devices from the position (the position of an electrostatic charger) where the electrostatic charge is given to the photosensitive member are different, respectively, when the distances are measured along the peripheral direction of the photosensitive member. The distance from the charging position to the developing device having yellow toner is shortest; to the device having cyan toner, the next; then to the device having magenta toner, the next; and to the device having black toner, the longest. Because of the difference in distance, the time required for beginning the development by each of the developing devices after the photosensitive member is charged is different depending on the respective developing devices. Thus, the resultant amount of the dark attenuation of the photosensitive member at the time of the development differs inevitably. In other words, depending on the position of each developing device, the dark attenuation amount of the photosensitive member differs a the time of the development.
When the amount of the dark attenuation differs as in this case, the density of the toner image after the development differs even if the amount of the electrostatic charge provided by the charger is the same; hence making it difficult to obtain high-quality images.
In order to avoid this drawback, the applicant hereof has proposed as given below. In other words, in Japanese Patent Laid-Open No. 61-120175, a provision of a controlling means is proposed so as to control and vary the output of an electrostatic charger in accordance with the developing device selected from the plural developing devices (the difference in the dark attenuation due to the difference in its position). Also, in Japanese Patent Laid-Open No. 61-117572, a provision of a image density adjusting mean is proposed so as to modify and adjust the developing bias voltage or the image exposure amount per developing device in accordance with the amount of the dark attenuation of the photosensitive member. With this image density adjusting means, it is possible to obtain the same level of image density at all times under a given condition of density adjustment set by an operator.
However, according to the conventional examples set forth above, there are some cases where the gradient tonality of the intermediate tone of an original image becomes different even when the image density is regular. Particularly, in a color image recording apparatus or the like for which a severe reproduction of the intermediate tone is demanded, there is a possibility that this difference in the tonality leads to a defective image.
Further, the dark attenuation characteristic of a photosensitive member varies as the time elapses. Consequently, even if the density adjustment including the intermediate tonality is properly made at the outset of using an image recording apparatus, the intermediate tones will become imbalanced as the apparatus is in use for a long period of time.